A typical method of retrieving data similar to retrieval data from among a retrieval target data series, including video data and audio data stored in a storage device, includes calculating similarities between the retrieval data and all pieces of data in the retrieval target data series, and comparing them with a threshold. However, as the amount of calculation for similarities between pieces of data is generally large, the above method in which similarities between the retrieval data and all pieces of data in the retrieval target data series must be calculated needs a long time for retrieval. As such, some methods for speeding up this type of retrieval have been proposed.
For example, Patent Document 1 describes, in the background art section, a method of performing retrieval at a high speed in such a manner that similarity calculation is terminated if a similarity exceeds a certain threshold. Patent Document 1 also proposes a method of calculating similarities between a part of data series and another one of or a plurality of parts as a self similarity table, and using the table to perform retrieval at a high speed.